1. Field of the Invention
This invention relates generally to a software calibration strategy for an electrohydraulic transmission control system and, more particularly, to a software calibration strategy for calibrating solenoids, solenoid controlled valve bodies, or solenoid controlled valve bodies with integrated control units that are used to control an automatic transmission for a vehicle.
2. Discussion of the Related Art
Automatic vehicle transmissions employ a plurality of gears, synchronizers, brake bands and clutches that are electronically controlled to automatically shift the gears of the vehicle. Typically, solenoid controlled valves are employed within the transmission to provide hydraulic pressure to control the operation of the various components and systems therein, such as clutches, brake bands, lubricant flow, hydraulic pressure, etc. The valves are generally proportional valves in that the amount of current applied to the solenoid of the valve determines the hydraulic pressure at the control output of the valve.
A solenoid controlled valve includes a coil wound around an armature and a valve body. The solenoid and the valve body can be attached or separate. The valve body is coupled to a hydraulic input line, a hydraulic exhaust line and a hydraulic control output line. Hydraulic supply pressure is applied to the input line. The supply pressure applied to the valve is stable, and the control pressure on the output line of the valve is set by the force of the armature. In a normally low (exhausted) solenoid. When no current is applied to the coil, the control output line is coupled directly to the exhaust line so that no output pressure is applied to the control line. When current is applied to the solenoid, the armature moves so that the hydraulic supply pressure is directed to the control line. The amount of current applied to the coil sets the force on the armature, and thus its ability to maintain its regulated output pressure. The valve control pressure feedback force positions the valve to maintain the precise control pressure that balances the force of the armature. This provides a specific control pressure for each current setting even when the load changes in the control pressure circuit.
Proportional solenoid controlled valves need to be calibrated so that they provide the desired control pressure relative to the input current. If the control valves in the transmission are not properly calibrated, then the performance of the transmission is reduced by a variety of factors, such as lower fuel economy, hard shifts, etc. Typically, the valve is calibrated by mechanical devices when the valve is manufactured. One known mechanical device is a threaded mechanism that increases or decreases a spring force applied to the armature. A second mechanical technique for calibrating a solenoid or solenoid piloted valve is to selectively adjust the working air gap between the armature and pole piece of the solenoid, which adjusts the magnetic force level of the device.
The known mechanical techniques of calibrating a solenoid valve are typically costly because the calibration components are expensive to manufacture and the calibration process is time consuming. Further, the equipment required to perform the calibration is expensive and has inherent accuracy limitations. Additionally, the solenoid or solenoid piloted valve is not calibrated to other system components, such as electronics, other valves, leakage, etc, that may affect the valve performance when it is employed in the system.